Conventionally known glow plugs used to assist the start of an internal combustion engine include a glow plug having a built-in sensor, which is a combustion pressure sensor for detecting the combustion pressure of an internal combustion engine (refer to Japanese Patent Application Laid-Open (kokai) No. 2005-90954); a sensor for detecting the temperature of a heater of the glow plug; a sensor for detecting ion current (refer to Japanese Patent Application Laid-Open (kokai) No. 10-122114); a sensor for detecting combustion light; or a like sensor.
Meanwhile, a glow plug having a built-in sensor of a certain kind requires not only a lead wire for energizing a heater of the glow plug but also a single or a plurality of sensor connection lines for transmitting an output signal from the sensor and driving the sensor. Such a glow plug having a built-in sensor may be configured such that the sensor connection line(s) and the heater-energizing lead wire extend rearward from the rear end of the glow plug (from an end axially opposite the heater located at the front end of the glow plug).
Furthermore, for protection against entry of moisture and oil droplets from the outside, the glow plug may require liquid-tight sealing of its rear end by use of a grommet which is formed from rubber-like elastic material and which allows the heater-energizing lead wire and the sensor connection line(s) to extend through respective insertion holes formed in the grommet.
In some cases, a glow plug having a built-in sensor may require connection to an external power supply unit via a heater-energizing lead wire. This heater-energizing lead wire may be relatively large in the cross-sectional area of its conductor and thus have a large outside diameter as measured to include its coating layer. For example, a glow plug having a built-in sensor may require quick raising of temperature; for example, may require raising of its temperature to about 1,000° C. in 2 or 3 seconds. In order to externally apply a large current to a heater conductor, such a glow plug uses a heater-energizing lead wire which is relatively large in the cross-sectional area of its conductor and has a large outside diameter as measured to include its coating layer. As for a sensor connection line, in many cases, a sensor connection line having a relatively small outside diameter will suffice, since what is required of the sensor connection line is to transmit an output of a sensor and to supply a small power for driving the sensor.
However, in a glow plug having a built-in sensor configured such that a sensor connection line(s) and a heater-energizing lead wire extend rearward from the rear end of the glow plug (from an end axially opposite a heater located at the front end of the glow plug) and having a grommet that is used to establish liquid-tight seal, if the heater-energizing lead wire is large in the cross-sectional area of its conductor and thus has a large outside diameter as measured to include its coating layer, the outside diameter of the grommet must be increased in order to maintain liquid tightness. As a result, the outside diameter of the glow plug must also be increased. Meanwhile, because of demand for reduction in size and weight of an internal combustion engine, a reduction in diameter of a glow plug has been demanded. Therefore, increasing outside diameter is difficult for a glow plug and a grommet.
That is, for a glow plug having a built-in sensor configured such that a sensor connection line(s) and a heater-energizing lead wire extend rearward from the rear end of the glow plug and such that a grommet is used to establish liquid-tight seal, it is difficult to maintain or reduce the diameter of the glow plug while the cross-sectional area of a conductor of the heater-energizing lead wire is increased.